The testing question about carrier aggregation
Carrier (or channel) aggregation, which is part of the 3GPP Release 10, represents a highly effective way to dramatically increase the transmission bandwidth as LTE-Advanced is adopted. This process makes it possible for multiple conventional LTE component carriers on the PHY layer, as opposed to single carriers, to be used together to ramp up supported data rates, reduce latency and enhance spectrum efficiency.
The upshot is that mobile operators can leverage carrier aggregation in order to provide a more comprehensive range of data-intensive multimedia applications, with full backward compatibility with existing hardware being maintained and no additional costs needing to be incurred.
To achieve the desired boost in data transfer, an increase in the total transmission bandwidth compared to what can be taken care of by a single LTE component carrier is called for. The objective is to attain throughput rates of 1Gbps across the downlink and 500Mbps across the uplink by using the compounded capacity of up to five 20MHz wide component carriers at once. Trials carried out in Southeast Asia are already managing to surpass this figure.
Operators will be able to benefit from dynamic load balancing of mobile traffic over these carriers. Asymmetric operation will be required, with more carriers potentially needed in one direction than in the other.
Carrier aggregation permits greater operational agility when it comes to allocating available spectrum as the required bandwidth can be cobbled together using a number of different component carriers. It facilitates better distribution of RF coverage, as the presence of additional carriers will allow a bolstering of data rates at the heart of the cell. In addition, it means that poor coverage at the perimeter of cell can be improved.
The allocation of the spectrum can be either contiguous (with no frequency gaps between them) or non-contiguous (with gaps of several hundred kHz permissible). Contiguous is obviously the simplest form of carrier aggregation. Non-contiguous carrier aggregation can be done through the use of carriers all located in the same frequency band or in several different bands.
This piecemeal approach is almost certain to be needed, but the potential downside of non-contiguous carrier aggregation is that interference between adjacent component carriers could affect the bit error rate and thus impact on signal integrity. Furthermore, this necessitates that mobile handsets have a separate transceiver for each carrier.
Given that the fragmentation of frequency bands is almost inevitable, a non-contiguous aggregation across several different bands is certain to see widespread use. The variations permitted in carrier aggregation are likely to lead to a rise in the sophistication seen in mobile handsets.
Operators will require access to test mobiles that are fully equipped to deal with carrier aggregation long before the actual mobile handsets become commercially available. They will predominantly concentrate on the handset’s capability to handle the large quantity of data resulting from simultaneous transfer of data over multiple receiver chains.
Engineers will need to employ test kits that have an array of network test and verification functions, with the ability to generate multiple component carriers at the same time and analyze the performance of each of the resulting transmit and receive chains. This will be compounded greatly if carriers are in different spectral frequency bands, as synchronization problems are likely to arise in this case.
Time alignment errors should not exceed 1.3μs for carriers in the same band and 130ns for carriers situated in separate bands. The asymmetric nature of the mechanism again needs to be provisioned for within the test equipment being procured.
Carrier aggregation should prove extremely effective in expanding deliverable mobile bandwidth. It will increase the overall capacity of the network and expedite a reduction in latency, too, without coexistence with legacy network technologies being an obstacle. It will allow weak RF performance at the edge of the cell to be tackled, as well as helping to address areas where there is very high bandwidth demand.
The innate flexibility that characterizes this technique will also be a key advantage to mobile operators over the coming years as they look to make the most of network resources they have already deployed and curb further investment. In order to benefit from it, however, operators and their technology contractors will need to source cutting-edge, multi-channel test solutions.
Nadaraj Govindasamy is director of Livingston’s Southeast Asian operations